In the preparation of photomechanical printing plates black-and-white halftone colour separation prints are used as photomasks in the production of printing plates for printing with subtractive colour printing inks. Inks of the subtractive colours cyan, magenta and yellow are used, as well as a certain amount of black ink applied by the black printer. Because of the overlapping spectral absorption bands of the subtractively coloured inks, the use of printing plates produced from photographic colour separation halftones without any correction usually results in poor quality colour reproduction. Accurate reproduction normally involves corrections for hue, saturation and lightness values. Apart from colour corrections necessary for faithful colour reproduction, colour modifications are often necessary for special effects, for example for purely aesthetic reasons or in order to achieve a heightened visual impact of selected matter in printed advertisements.
The amount and distribution pattern of any one of the printing inks in a multicolour print depends on the dot size distribution in the corresponding printing plate, which distribution in turn depends on the dot size distribution in the black-and-white halftone colour separation directly used in the making of the plate.
It is common practice to make a pre-press colour proof to evaluate the quality of the halftone colour separations. Suitable colour proofing processes for that purpose are described in e.g. GB-P 1,264,313, U.S. Pat. Nos. 4,701,401 and 4,553,835.
It is known to effect colour corrections or modifications by producing a contact-print of the initial halftone colour separation using an appropriately controlled exposure dose in an area to be dot-size corrected. A print wherein the dots in said area are smaller or larger than those of the initial separation can be produced, which print can then be directly used in making the corresponding printing plate. This technique for colour-correction or modification wherein dot-sizes are changed photographically and specifically in the areas where modification is required by controlled over-exposure or under-exposure is known in the art as "dry dot etching" or "dry etching" (see e.g. Research Disclosure July 1983 item 23135).
In dry dot etching for affecting a colour change in any selected area of an image, as determined for example from an examination of a colour proof, at least one isolation masks are made for use in combination with one or more of the initial halftone separations when making the aforesaid contact prints. If colour change is required in an area of simple shape, a suitable mask can be produced from a so-called cut-and-peel film or by hand-lacquering a mask film (see e.g. DE-P 3 140 955. For more complicated shapes it is much better to produce the mask photographically.
As described in the above mentioned Research Disclosure and likewise in published European Patent Application 0 244 241 a mask serving the purpose of dot size correction or modification in a single halftone colour separation is obtained photographically by making an overlay combination of several halftone separation positives and/or negatives so that on photographing that overlay on a photographic silver halide emulsion material having a transparent base the colours to be corrected appear transparent on the mask. In order to select the required separations for constituting the overlay sandwich use is made of a colour chart wherefrom for each colour to be identified as a transparent area in the final mask a set of maximum three overlay films is found that fulfill the requirement for mask production.
Since it is necessary to produce a transparent mask area free from any dot structure a diffuse exposure of said sandwich is applied, e.g. by using in contact exposure a more or less mat interlay film (diffusion sheet) between the selected halftone separations and the film wherein the mask has to be produced.
For example, the colour "red" is isolated by photographing accurately in register the cyan positive separation with the magenta and yellow negative separation images on a mask film. One or more light-diffusing sheets are arranged in conjunction with the halftone colour separations to suppress the halftone dot structure in the mask. The thus exposed and developed mask film provides light shielding except for in the red coloured areas, to be corrected or modified in dot size.
The actual tonal correction takes place, in that the colour separation to be corrected is copied alone using a first exposure and is copied in combination with the mask onto a suitable halftone reproduction type film, i.e. a film having a very steep gradation characteristic yielding sharp screen dot reproduction.
The above method of mask production is well suited for the correction of tonal values of relatively pure colours that are easily identifyable but fails when non-pure colours, so-called tertiary colours formed by a mixture of all three basic primary colours (red, green and blue), such as brownish have to be isolated and corrected.
Since it is not uncommon for colour corrections or modifications to be required in several distinct areas of a halftone separation image and the production of separate masks for each area is a very time consuming job it would be particularly advantageous if all areas to be corrected against their backgrounds could be isolated in a one-step overlay procedure.